Bio-oil is a potential renewable energy source for electricity and heat generation as well as being an alternative transportation fuel. However, several hurdles must be crossed before bio-oil can be used reliably. One of the main issues is storage stability of the oils. During storage, there is potential for bio-oils to undergo changes due to oxidative and thermal degradation. Oxidation can lead to polymerization and significantly increased viscosity. Thermal degradation causes partial decomposition of bio-oil constituents and can lead to the loss of volatiles. Both degradative factors can lead to viscosity and compositional changes (Diebold & Czernik (1997) Energy Fuels 11: 1081-1091; Diebold, J. P. (1999) NREL/SR-570-27613; Oasmaa & Kuoppala, (2003) Energy Fuels 17: 1075-1084; Boucher et al., (2000) Biomass Bioenergy 19: 351-361).
Most applications for bio-oils require that bio-oils retain the favorable initial physical properties during storage, shipment and use (Diebold, 1999) to avoid the risk that filters, injectors, input lines, etc, may become obstructed. In addition, the high level of reactive species and water content of bio-oil makes it unstable under normal storage conditions, which leads to increased viscosity. In addition, high oxygen and water content also lower the heating value of the fuel (Oasmaa & Kuoppala, (2003) Energy Fuels 17: 1075-1084).
During the aging process, bio-oil viscosity and the chemical composition changes dramatically, mainly due to polymerization reactions (Adjaye et al., (1992) Fuel Processing Technol. 31: 241-256). A higher degree of polymerization results in an increase in viscosity. Polymerization reactions that lead to viscosity increases are accelerated at higher storage temperatures and it has been shown that the rate of change in viscosity can increase from 0.009 cP/day when stored at −20° C. to more than 300 cP/day at 90° C. (Adjaye et al., (1992) Fuel Processing Technol. 31: 241-256). Adding solvents after pyrolysis can increase the stability of bio-oil during aging. Diebold & Czernik ((1997) Energy Fuels 11: 1081-1091) showed that solvent addition could significantly decrease viscosity changes during aging. Solvents used in the study included ethyl acetate, methyl isobutyl ketone and methanol, acetone, methanol, acetone and methanol, and ethanol. Their findings showed that methanol at 10 wt % enhanced bio-oil stability most effectively, and reported a reduction in the rate of change in viscosity.
The immediate effects of adding an alcohol are decreased viscosity and increased heating value (Oasmaa et al., (2004) Energy and Fuels 18: 1578-1583; Moens et al., (2009) Energy Fuels 23: 2695-2699; Stamatov et al., (2006) Renewable Energy 31: 2108-2121). These improvements to bio-oil made it more favorable for combustion applications such as in furnaces, boilers and gas turbines, or as an alternative to diesel where untreated bio-oils can require major changes in current systems (Gust S., (1997) In: Bridgewater & Boocock., eds: Developments in thermal biomass conversion. London: Blackie Academic & Professional pp 481-488; Stamatov et al., (2006) Renewable Energy 31: 2108-2121). The increase in heating value for bio-oils mixed with ethanol is due to the fact that ethanol has a high heating value of 27 MJ kg−1, which is higher than that of most bio-oils.
Most studies have directly added alcohols after pyrolysis (Moens et al., (2009) Energy Fuels 23: 2695-2699; Oasmaa et al., (2004) Energy and Fuels 18: 1578-1583, Diebold & Czernik ((1997) Energy Fuels 11: 1081-1091) which works well to increase stability and heating value. However, several recent studies (Zhang et al., (2006) Energy & Fuels 20: 2717-2720; Tang et al., (2008) Energy Fuels 22: 3484-3488; Mahfbd et al., (2007) Transactions of the Institution of Chemical Engineers Part B 85, 466-472; Chang et al., (2007) Chinese Chemical Letters, 18: 445-447; Junming et al., (2008) Biomass and Bioenergy 32: 1056-1061; Lu et al., Chin. Chem. Lett. (2007) 18: 445-447), showed that using reactive distillation of bio-oil with alcohol along with an acid or base catalyst, esterification of bio-oil was possible. Esterifying bio-oil can significantly improve the quality of bio-oil by lowering water content, viscosity and free-acid content. Additional improvements in bio-oil quality include an increase in heating value by as much as 50% (Junming et al., (2008) Biomass and Bioenergy 32: 1056-1061; Zhang et al, (2006) Energy Fuels 20: 2717-2720) and an increase in stability due to the removal of acids that catalyze many polymerization reactions. Junming et al. (2008) showed that after three months of aging, esterified bio-oil exhibited very little viscosity increase. Ji-lu et al. (J. Anal. Appl. Pyrolysis (2007) 80: 30-35) introduced well-sprayed ethanol into a bio-oil condenser as a precursor to spraying bio-oil once enough was produced. The intent was to quickly cool vapors to prevent polymerization reactions, though esterification was not observed.
Fischer esterification is proposed to be the reaction pathway in conversion to esters. The esterification reaction follows the equation:RCOOH+CnH2n+1OH⇄RCOOCnH2n+1+H2O,leading to the formation of water and an ester. The simplest ester that can be produced is methyl formate, HCOOCH3, when methanol (CH3OH) is used as the alcohol and is reacted with formic acid, HCOOH. Industrially the reaction is always catalyzed by a strong acid. Several studies (Junming et al., (2008) Biomass and Bioenergy 32: 1056-1061; Tang et al., (2008) Energy Fuels 22: 3484-3488) have used solid acid catalysts to enhance the bio-oil esterification reaction which improved bio-oil quality by increasing HHV and pH and reducing specific gravity, viscosity and water content for esterified bio-oil.
Esterification reactions are not, however, intended to be limited to linear, alkyl, saturated alcohols or acids. For example, and not intended to be limiting, branched alcohols such as tert-Bu-CnH2n+1OH) and acids (e.g., iso-Pr-CnH2n+1COOH), (poly)unsaturated species (e.g., CH3HC═CHCnH2n+1OH and CH3HC═CHCnH2n+1COOH), and aromatic compounds such as, but not limited to, C6H5OH, C6H5CH2OH, and C6H5COOH, and the like may also be used to carry out esterification reactions according to the present disclosure.
Several studies (Chu et al., (1996) Appl. Catal., A: General 145: 125-140; Kirumakki et al., (2006) Appl. Catal., A: General 299: 185-192; Koster et al., (2001) J. Catal. 204: 333-338; Miao & Shanks (2009) Appl. Catal., A: General 359: 113-120) have shown the potential to use heterogeneous catalysts for esterifying model bio-oil compounds such as acetic acid. As an example, Miao & Shanks ((2009) Appl. Catal., A: General 359: 113-120) esterified acetic acid, a model bio-oil compound, using a mesoporous catalyst. Acetic acid conversion was close to 40% at a 250 min reaction time at 50° C. using the catalyst. Zhang et al., ((2006) Energy Fuels 20: 2717-2720) esterified acetic acid in a reflux reactor and showed yields ranging from 15% (no catalyst) to 100% (solid acid catalyst). Koster et al., ((2001) J. Catal. 204: 333-338) and Chu et al., ((1996) Appl. Catal., A: General 145: 125-140) performed vapor-phase esterifications of acetic acid with ethanol. Koster et al., (2001) performed gas-phase esterifications over several mesoporous catalysts, and showed moderate ester yields (<25%). Equilibrium for the reaction lies far to the right, especially in the vapor phase, for which the thermodynamic equilibrium constant is 367 for the reaction of ethanol and acetic acid to form ethyl acetate.